You can type cd /Users/yourusername/Documents to go to your documents folder. cd is the terminal command to change the directory you're working in.

You can also type cd ~/Documents (~ is short for your home directory).

Finally, if you go to your home folder in the Finder, you can drag Documents onto the terminal window, and it will show the path to it. This is useful if you ever have a file or folder that you want to learn the path to.

This is all you need to do to run the jar file java -jar ~/Docuemnts/jar. This is really what you want to do, as this runs the file you've created and the cd ~/Ducuments/ command only sets the terminal to work in the Documents directory (ie: folder), but little else.